[nyreed]

We had this happen. There was leak somewhere in the pipe running from the street into our property, which ran under a small front grass lawn before running into the building. We had been away for a month and came back to find everything very damp and overly cold, it was quite subtle. We figured the leak was somewhere under the lawn and running downhill to the house.

So the man from the water company comes and says it's very hard to detect where the leak is, but he will come back with his dowsing rod to find it.

We told him not to bother, and figuring the leak was somewhere between the house and the water mains under the pavement (about 2m distance), started digging holes where the ground started getting soggier. Found it quickly by applying a bit of common sense.